1. Field of the Invention
The present invention generally relates to solder baths and, more particularly, is directed to a jet solder bath which is assembled in an automatic soldering apparatus in order to carry out soldering on a printed circuit board.
2. Description of the Related Art
A jet solder bath is an apparatus in which a nozzle and a pump are installed within a solder bath. A molten solder supplied via the pump into the solder bath is jetted upward from the nozzle and the printed circuit board is brought in contact with the molten solder thus jetted to thereby solder a predetermined portion.
The temperature of the molten solder within the solder bath is 200.degree. C. or more and, in addition, the molten solder is dropped into the solder bath after having been jetted upward so that the molten solder thus dropped and the molten solder within the bath are inevitably mixed. At that time, the molten solder is frequently exposed to the air and as a consequence, the molten solder is severely oxidized at the portion in which the molten solder is dropped. In the following description, the molten solder thus oxidized will be referred to as "oxide" for simplicity. The solder once oxidized loses a solder function and must be discarded, which causes a great loss from an economical standpoint.
Further, the oxide thus produced is collected near the nozzle mouth, i.e., in the nozzle mouth and the solder bath. If there are provided a primary nozzle and a secondary nozzle, the oxide is collected near the nozzle mouths thereof and also in the space between the respective nozzle mouths. If the oxide is accumulated, then the accumulated oxide is overflowed to the outside of the solder bath from the lower wall of the solder bath or the printed circuit board immersed into the molten soldering liquid within the solder bath is smudged by the accumulated oxide, thereby hindering the soldering on the printed circuit board. Furthermore, the accumulated oxide causes a failure in the soldering-process.
Furthermore, when the molten solder is dropped, peripheral devices are smudged by resultant solder splashes and therefore the peripheral devices are caused to malfunction. In some cases, the printed circuit board is smudged by solder splashes, which causes the failure in the soldering-process.
Accordingly, a jet solder bath in which a height from the jetted molten solder to the bath surface of the solder bath is reduced to suppress the solder from being oxidized, that is, a jet solder bath which can prevent the molten solder from being oxidized has been proposed.
In the conventional jet solder bath which can prevent the solder from being oxidized, solder tanks are provided at both sides of the nozzle to raise the liquid level at which the jetted molten solder is dropped. Thereby the height is reduced. Thus, a force in which the molten solder is dropped into the liquid surface is weakened, thereby reducing the amount of the solder oxide produced.
In the jet solder bath which is intended to prevent the molten solder from being oxidized, however, the molten solder is exposed to the air when the molten solder is jetted, which unavoidably causes the molten solder to be oxidized.
In addition, in order to obviate harmful influences brought about by the oxide, the oxide accumulated within the solder bath must be removed several times in a day. However, since the oxide is accumulated in the nozzle wall surface within the solder bath or in a very narrow place such as a spacing between the primary and secondary nozzles as earlier noted, it is very difficult to remove the oxide accumulated thereon.
To solve the above-mentioned problem, in the solder bath according to the prior art, the following proposal is made. That is, since most of oxide produced in the above solder tank is floated on the molten solder in the solder tank, the oxide together with the molten solder is dropped and collected into a second solder tank separately provided from an upper cutaway portion formed at one end or at the respective ends of the solder tank provided along the nozzle mouth. Then, in the second solder tank, the oxide is separated from the molten solder by the use of a proper filter or by making effective use of a difference of specific gravity between the oxide and the molten solder, and the molten solder from which the oxide is separated is recycled in the solder bath.
However, in a solder bath described in Japanese Laid-Open Patent Publication No. 61-82964, the molten solder must be flowed and dropped from the above-mentioned cutaway portion in order to remove the oxide floating from the solder tank provided along the nozzle mouth, which unavoidably allows the molten solder to be oxidized additionally in that portion.